In many situations and circumstances, it is important to know if the insulation surrounding a wire or cable has deteriorated. These situations are not limited to those in an earth environment, but include satellites and space stations in the vacuum environment of space. Many of the devices and methods used to detect defects in electrically insulated wires cannot be used in the vacuum of space since they require the use of a conductive or semi-conductive gas which contains ions. See U.S. Pat. Nos. 3,096,478; 3,263,165; and 3,639,831.
It would be desirable, therefore, to have a localized source of electrons that can be positioned and electrically scanned along the length of an insulated cable which is located in a vacuum to assist in detecting and pinpointing defects in the insulation while the cable is operating.